A photocatalyst generally refers to a material which causes strong oxidation-reduction by light energy and has semiconductor properties. When energy in a specific region is applied to a semiconductor, atoms of the semiconductor are excited from the valence band to the conduction band. Here, electrons (e−) are formed in the conduction band and holes (h+) are formed in the valence band. These electrons and holes cause various reactions such as decomposition of organic materials through strong oxidation or reduction. Such reactions are utilized to sterilize, antibacterialize, decompose, deodorize, and collect substances adhering to a surface of a material or contaminants in air or a solution, and photocatalysts using such reactions have been used in a variety of fields including cooler fillers, glass, tiles, outer walls, foods, inner walls of a factory, metal products, water tanks, purification of marine pollution, fungus prevention, UV protection, water purification, atmosphere purification, and hospital infection prevention and are recently applied to the field of producing hydrogen from water due to their capability of accelerating decomposition of water.
Among such photocatalysts, titanium dioxide (TiO2) is most widely used due to its excellent photoactivity, chemical and biological stability, and durability. However, titanium dioxide exhibits photocatalytic properties only in the UV region. UV light accounts for only about 3% (32 W/m2) of the total amount of sunlight (1004 W/m2), and, in cloudy weather, the atmosphere transmits even less UV light due to low transmittance of cloud.